Group III nitride compound semiconductor device and method for forming an electrode

ABSTRACT

An AlN buffer layer  2 ; a silicon (Si)-doped GaN high-carrier-concentration n +  layer  3 ; an Si-doped n-type Al 0.07 Ga 0.93 N n-cladding layer  4 ; an Si-doped n-type GaN n-guide layer  5 ; an active layer  6  having a multiple quantum well (MQW) structure, and including a Ga 0.9 In 0.1 N well layer  61  (thickness: about 2 nm) and a Ga 0.97 In 0.03 N barrier layer  62  (thickness: about 4 nm), the layers  61  and  62  being laminated alternately; an Mg-doped GaN p-guide layer  7 ; an Mg-doped Al 0.07 Ga 0.93 N p-cladding layer  8 ; and an Mg-doped GaN p-contact layer  9  are successively formed on a sapphire substrate. A p-electrode  10  is formed of a film of titanium nitride (TiN) or tantalum nitride (TaN) (thickness: 50 nm). The contact resistance of this electrode is reduced through heat treatment.

The present Application is a Divisional Application of U.S. patentapplication Ser. No. 10/239,895 filed on Feb. 19, 2003, now U.S. Pat.No. 6,806,571, which is a 371 of PCT/JP01/01178, filed Feb. 19, 2001.

TECHNICAL FIELD

The present invention relates to a device including an electrode whichhas low contact resistance to a p-type Group III nitride compoundsemiconductor, and which does not occur any chemical reaction such asoxidation as time passes. The present invention also relates to a methodfor forming the electrode. As used herein, the term “Group III nitridecompound semiconductor” refers to a semiconductor generally representedby the following formula: Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1), and examples thereof include binary semiconductors such asAlN, GaN, and InN; ternary semiconductors such as Al_(x)Ga_(1−x)N,Al_(x)In_(1−x)N, and Ga_(x)In_(1−x)N (in each case, 0<x<1); andquaternary semiconductors represented by the following formula:Al_(x)Ga_(y)In_(1−x−y)N (0<x<1, 0<y<1, 0<x+y<1). Unless otherwisespecified, in the present specification, the term “Group III nitridecompound semiconductor” also includes p-type or n-type Group III nitridecompound semiconductors doped with an impurity.

BACKGROUND ART

Group III nitride compound semiconductors have direct transition andwhose emission spectrum can be changed over a wide range from UV to redwhen used in a device such as a light-emitting device. Therefore GroupIII nitride compound semiconductor have been used in light-emittingdevices such as a light-emitting diodes (LEDs) and laser diodes (LDs).In addition, since a Group III nitride compound semiconductor has a wideband gap, a device employing the semiconductor is considered to beoperated reliably at high temperature, as compared with a deviceemploying a semiconductor other than a Group III nitride compoundsemiconductor. Therefore, applying group III nitride compoundsemiconductors to electron devices including an FET, have beendeveloped. Moreover, because arsenic (As) is not contained in Group IIInitride compound semiconductors as a main constitution element,application of the semiconductors to the various semiconductor deviceshas been expected from the environmental viewpoint.

In general, when a metallic layer is merely formed on the surface of acompound semiconductor, ohmic contact between the metallic layer and thecompound semiconductor fails to be obtained. Therefore, conventionally,the ohmic contact can be obtained by thermal treatment of the sample todiffuse the metal in the semiconductor. In the case of a p-type GroupIII nitride compound semiconductor, a resistivity of the p-type GroupIII nitride semiconductor is not reduce to the same level of that ofn-type semiconductor, even the sample is taken heat treatment process orelectron beam irradiation process. Therefore, the current does notspread in a lateral direction in the p-type layer, but flows just belowelectrade. Accordingly light is emitted merely from a portion directlybeneath the electrode. To solve this problem, there has been proposed acurrent-diffusing electrode which is formed by laminating a nickel (Ni)layer (thickness: some hundreds Å) and a gold (Au) layer (thickness:some hundreds Å) and performing heat treatment thereafter, whichexhibits light transmittance and ohmic characteristics (Japanese PatentApplication Laid-Open (kokai) No. 6-314822). However, this electrode oftwo-layer structure including a nickel (Ni) layer and a gold (Au) layerhas a contact resistivity as high as 2×10⁻³ Ωcm² when the electrodecontacts with a p-type Group III nitride compound semiconductor but theresistivity is still high. Therefore, a Group III nitride compoundsemiconductor device having this electrode still has a high operationvoltage.

DISCLOSURE OF THE INVENTION

The present inventors have previously applied for a patent regarding aninvention related to a p-electrode including a titanium (Ti) layer and atantalum (Ta) layer (Japanese Patent Application No. 10-202697). Thisp-electrode is superior to the aforementioned electrode of two-layerstructure including a nickel (Ni) layer and a gold (Au) layer in termsof initial contact resistivity, but there still remains room forimprovement of the p-electrode. That is, when the p-electrode includinga titanium (Ti) layer and a tantalum (Ta) layer is exposed to air forone week, the contact resistance of the electrode increases by a factorof about 1,000. The reason for the increase in contact resistance isthought to be as follows: the two-metallic-layer electrode is oxidizedby oxygen and moisture contained in air; or a metal nitride is formed bynitrogen contained in a Group III nitride compound semiconductor whichis in contact with the electrode. As a result, the ohmic contact can notkeep low contact resistance.

The present invention has been accomplished in order to solve theaforementioned problems. An object of the present invention is toprovide an electrode which has low contact resistance to a p-type GroupIII nitride compound semiconductor, and which does not occur anychemical reaction such as oxidation as time passes.

In order to solve the aforementioned problems, a first feature of theinvention can be employed. Through use of this feature, a memberselected from the group consisting a titanium nitride (TiN_(x))electrode, a tantalum nitride (TaN_(x)) electrode, and a tantalumtitanium nitride (Ta_(y)Ti_(1−y)N_(z)) electrode is formed on a p-typeGroup III nitride compound semiconductor. The titanium nitride (TiN_(x))electrode, tantalum nitride (TaN_(x)) electrode, or tantalum titaniumnitride (Ta_(y)Ti_(1−y)N_(z)) electrode formed on the p-type Group IIInitride compound semiconductor is not oxidized by oxygen or moisturecontained in air, and is not chemically reacted with nitrogen (N) atomscontained in the Group III nitride compound semiconductor which is incontact with the electrode. Therefore, the characteristics of thetitanium nitride (TiN_(x)) electrode, tantalum nitride (TaN_(x))electrode, and tantalum titanium nitride (Ta_(y)Ti_(1−y)N_(z)) electrodedo not vary as time passes. A reduction of the contact resistancegreatly contributes to suppression of generating heat in a semiconductordevice, and improving the life time of the device.

A second feature of the invention includes heat treatment of a memberselected from the group consisting the tantalum nitride (TaN_(x))electrode, titanium nitride (TiN_(x)) electrode, and tantalum titaniumnitride (Ta_(y)Ti_(1−y)N_(z)) electrode at 700 to 1,000° C. afterdeposition of the electrode. Because the tantalum nitride (TaN_(x))electrode, titanium nitride (TiN_(x)) electrode, or tantalum titaniumnitride (Ta_(y)Ti_(1−y)N_(z)) electrode is alloyed with the Group IIInitride compound semiconductor which is in contact with the electrodethrough this heat treatment, the value of the contact resistance can befurther reduced.

A third feature of the invention is a method for forming a p-electrodeof a device including a p-type Group III nitride compound semiconductor,the method comprising forming the p-electrode made of a member selectedfrom the group consisting tantalum nitride (TaN_(x)), titanium nitride(TiN_(x)), and tantalum titanium nitride (Ta_(y)Ti_(1−y)N_(z)) bysputtering. Such metal nitride electrode can be formed readily bysputtering.

A fourth feature of the invention includes reactive sputtering by use ofa mixing gas of nitrogen and a rare gas. By employment of a mixing gasof nitrogen and a rare gas, nitrogen atoms for forming a metal nitridecan be readily generated, and thus such a metal nitride electrode can beformed more readily.

A fifth feature of the invention includes heat treatment at 700 to1,000° C. after sputtering. Through heat treatment, the metal nitrideelectrode is alloyed with the Group III nitride compound semiconductorwhich is in contact with the electrode, and therefore, contactresistance can be further reduced.

The present invention can be carried out with reference to the followingdescription.

A metal nitride electrode is preferably formed of electricallyconductive titanium nitride (TiN) or tantalum nitride (TaN). Theelectrode may be formed of zirconium nitride (ZrN), niobium nitride(NbN), or tungsten nitride (WN). Furthermore, the electrode may beformed of a mixture of these metal nitrides. The metal nitride electrodemay be formed by, for example, reactive sputtering. In the case ofsputtering, introduction of nitrogen (N) atoms is required. Introductionof nitrogen atoms can be realized through reaction in a gas mixture ofnitrogen (N₂) and an inert gas such as a rare gas (e.g., helium (He),neon (Ne), argon (Ar), or krypton (Kr)). After formation of theelectrode, heat treatment is preferably carried out at 700 to 1,000° C.,more preferably at 800 to 950° C.

When Group III nitride compound semiconductor layers are successivelyformed on a substrate, the substrate may be formed of an inorganiccrystal compound such as sapphire, silicon (Si), silicon carbide (SiC),spinel (MgAl₂O₄), ZnO, or MgO; a Group III–V compound semiconductor suchas gallium phosphide or gallium arsenide; or a Group III nitridecompound semiconductor such as gallium nitride (GaN). The Group IIInitride compound semiconductor layers are preferably formed bymetal-organic chemical vapor deposition (MOCVD) or metal-organic vaporphase epitaxy (MOVPE), but may be formed by molecular beam epitaxy(MBE), halide vapor phase epitaxy (Halide VPE), or liquid phase epitaxy(LPE). The Group III nitride compound semiconductor layers may be formedby different growth methods.

For example, when layers of a Group III nitride compound semiconductorare formed on a sapphire substrate, in order to obtain a high qualitysimple crystal, it is preferable to form a buffer layer so as tocompensate lattice mismatching with the sapphire substrate. It is alsopreferable to provide a buffer layer when using the different substrate.As a buffer layer, a Group III nitride compound semiconductor which isformed at a low temperature such as Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1,0≦y≦1, 0≦x+y≦1) and more preferably Al_(x)Ga_(1−x)N (0≦x≦1) is used.This buffer layer may be a single layer or multiple layers havingdifferent compositions. The buffer layer may be formed at a lowtemperature of 380 to 420° C., or the buffer layer may be formed byMOCVD at a temperature in the range of 1,000 to 1,180° C. Alternatively,a buffer layer comprising AlN can be formed by reactive sputtering usinga DC magnetron sputtering apparatus and high purity metallic aluminumand nitrogen gas are used for source material. Similarly, a buffer layerexpressed by the general formula Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1, with the composition ratio being arbitrary) can be formed.Furthermore, it is possible to use the vapor deposition method, the ionplating method, the laser abrasion method, or the ECR method. Formationof the buffer layer by physical vapor deposition is preferably carriedout at a temperature in the range of 200 to 600° C. More preferably itis carried out at a temperature in the range of 300 to 600° C. and stillmore preferably in the range of 350 to 450° C. When a physical vapordeposition method such as these sputtering methods is used, thethickness of the buffer layer is preferably in the range of 100 to 3,000Å. More preferably it is in the range of 100 to 400 Å, and mostpreferably it is in the range of 100 to 300 Å. A multi layer maycomprise, for example, alternating Al_(x)Ga_(1−x)N (0≦x≦1) layers andGaN layers. Alternatively, a multi layer may comprise alternating layersof the same composition formed at a temperature of not higher than 600°C. and at a temperature of not lower than 1000° C. Of course, thesearrangements may be combined. Also, a multi layer may comprise three ormore different types of Group III nitride compound semiconductorsAl_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1). Generally, a bufferlayer is amorphous and an intermediate layer is singlecrystalline.Repetitions of unit of a buffer layer and an intermediate layer may beformed, and the number of repetitions is not particularly limited. Thegreater the number of repetitions, the more improvement incrystallinity.

A portion of Group III elements of a buffer layer and that of a GroupIII nitride compound semiconductor formed on the buffer layer may bereplaced with boron (B) or thallium (Tl), or a portion of nitrogen (N)atoms of a buffer layer or that of a Group III nitride compoundsemiconductor formed on the buffer layer may be replaced with phosphorus(P), arsenic (As), antimony (Sb), or bismuth (Bi). Also, the bufferlayer and the Group III nitride compound semiconductor may be doped withany elements which cannot express the composition thereof. For example,a Group III nitride compound semiconductor which is represented byAl_(x)Ga_(1−x)N (0≦x≦1) and which does not contain indium (In) andarsenic (As) may be doped with indium (In), which is larger in atomicradius than aluminum (Al) and gallium (Ga), or arsenic (As), which islarger in atomic radius than nitrogen (N), to thereby improvecrystallinity by compensation, that is, the expansion strain by dopedlarge atom compensates the compressive strain by nitrogen vacancy. Inthis case, since acceptor impurities easily occupy the positions ofGroup III atoms, p-type conductivity can be obtained as grown. Throughthe thus-attained improvement of crystallinity combined with thefeatures of the present invention, dislocation density can be morereduced to approximately 1/100 to 1/1000. In the case of an underlyinglayer comprising two or more repetitions of a buffer layer and a GroupIII nitride compound semiconductor layer, the Group III nitride compoundsemiconductor layers are further preferably doped with an elementgreater in atomic radius than a predominant component element. In thecase where a light-emitting device is a target product, use of a binaryor ternary Group III nitride compound semiconductor is preferred.

When an n-type Group III nitride compound semiconductor layer is formed,a Group IV or Group VI element, such as Si, Ge, Se, Te, or C, can beused as an n-type impurity. A Group II or Group IV element, such as Zn,Mg, Be, Ca, Sr, or Ba, can be used as a p-type impurity. The same layermay be doped with a plurality of n-type or p-type impurities or dopedwith both n-type and p-type impurities.

Optionally, dislocations of a Group III nitride compound semiconductorlayer may be reduced through lateral epitaxial overgrowth. In order toreduce dislocations, lateral epitaxial growth. Here a mask may beperformed, or trenches formed through etching may be filled throughlateral epitaxial growth.

An etching mask may be formed of a single film or a multi-layer filmformed from a polycrystalline semiconductor such as polycrystallinesilicon or a polycrystalline nitride semiconductor; an oxide or anitride, such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),titanium oxide (TiO_(x)), or zirconium oxide (ZrO_(x)); or a metal ofhigh melting point, such as titanium (Ti) or tungsten (W). The film maybe formed through any known method, such as a vapor-growth method (e.g.,deposition, sputtering, or CVD).

When etching is carried out, reactive ion-beam etching (RIBE) ispreferably performed, but the etching method is not limited to reactiveion-beam etching. Trenches having a V-shaped cross section (i.e.,trenches having no bottom surface) may be formed through anisotropicetching, so as not to form trenches having inner walls perpendicular toa substrate.

A semiconductor device such as an FET or a light-emitting device may beformed on a Group III nitride compound semiconductor. When alight-emitting device is formed, a light-emitting layer may have amultiple quantum well (MQW) structure, a single quantum well (SQW)structure, a homo junction structure, a hetero junction structure, or adouble hetero junction structure. The light-emitting layer may contain apin junction or a pn junction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relation between heat treatment temperature and contactresistance using a TiN electrode formed on a p-GaN layer.

FIG. 2 shows the relation between heat treatment temperature and contactresistance using a TaN electrode formed on a p-GaN layer.

FIG. 3 is a schematic diagram showing the structure of a Group IIInitride compound semiconductor device of a first embodiment of thepresent invention.

FIG. 4 is a schematic diagram showing the structure of a Group IIInitride compound semiconductor device of a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detail.

An aluminum nitride (AlN) layer was formed on a sapphire substrate whosecrystal plane is an A-plane; a gallium nitride (GaN) layer (thickness: 4μm) was formed on the AlN layer; and a magnesium (Mg)-doped p-GaN layer(thickness: 0.5 μm) was formed on the GaN layer. The p-GaN layer had ahole concentration of 7×10¹⁷ cm⁻³ and a resistivity of 5 Ωcm.

A TiN electrode or TaN electrode was formed on the p-GaN layer tomeasure a contact resistance in the shape which had a center portion andan annular portion surrounding the center portion. The distance betweenthe center portion and the annular portion was 8 μm. The metal nitrideelectrode was formed by reactive sputtering employing a gas mixture ofargon (Ar) and nitrogen (N₂) (ratio, 4:1). Sputtering was performed at100 W and 0.67 Pa. The values of the contact resistance of theas-deposited TiN electrode and TaN electrode, the contact resistanceafter heat-treatment at 460° C. to 800° C. for 10 to 40 minutes areshown in FIGS. 1 and 2, respectively. Here heat treatment was carriedout under high vacuum (i.e., 1.3×10⁻⁶ Pa). In X-ray diffractionanalysis, the thus-formed TiN electrode and TaN electrode were found toexhibit a peak corresponding to a (111) plane and a peak correspondingto a (11-20) plane, respectively. It is considered from the results thatthe metallic atom is trivalent and the metallic atom isstoichiometrically bonded to a nitrogen atom at a 1:1 ratio.

FIG. 1 shows the relationship between heat treatment temperature and thecontact resistance of the TiN electrode. As shown in FIG. 1, when heattreatment is carried out at 460° C. for 10 minutes, the contactresistance of the electrode is reduced to about 1/10 compared with thatof the as-deposited electrode, but the value thereof of the thus-heatedelectrode is still very high, as high as about 10¹⁰ Ω. In contrast, whenheat treatment is carried out at 720° C. for 10 minutes, the contactresistance of the electrode is considerably reduced to about 10⁸ Ω, andwhen heat treatment is carried out at 800° C. for 30 minutes, the valueof the contact resistance of the electrode is considerably reduced toabout 2×10⁵ Ω.

FIG. 2 shows the relationship between heat treatment temperature and thevalue of the contact resistance of the TaN electrode. As shown in FIG.2, the as-deposited TaN electrode before the heat-treatment has aresistance of about 10¹¹ Ω. When heat treatment is carried out at 460 to630° C. for 10 or 20 minutes, the contact resistance of the electrode isreduced to 4×10⁸ to 6×10⁹ Ω but a remarkable reduction of resistance isnot observed. In contrast, when heat treatment is carried out at 720° C.for 10 or 20 minutes, the contact resistance of the electrode isconsiderably reduced to about 10⁷ Ω, and when heat treatment is carriedout at 800° C. for 20 or 40 minutes, the contact resistance of theelectrode is more remarkably reduced to 2×10⁵ to 3×10⁵ Ω.

The contact resistance of the TiN electrode or TaN electrode increasesonly by a factor of about 10 or less after five days pass. That is, ascompared with the case of the aforementioned titanium (Ti)/tantalum (Ta)two-layer electrode in which the contact resistance increases by afactor of 1,000 or more after six days pass, the above-formed electrodeexhibits very remarkable improvement.

Embodiments of the present invention which is applied to light-emittingdevices will next be described. FIG. 3 is a schematic diagram ofcross-sectional view of a laser diode 100 according to a firstembodiment of the invention, which includes a GaN-based compoundsemiconductor formed on a sapphire substrate 1.

As shown in FIG. 3, the laser diode 100 includes the sapphire substrate1, and an AlN buffer layer 2 (thickness: 30 nm) is formed on thesapphire substrate 1. On the buffer layer 2 are successively formed asilicon (Si)-doped GaN high-carrier-density n⁺ layer 3 (thickness: about5 μm, electron density: 1×10¹⁸/cm³); a silicon (Si)-dopedAl_(0.07)Ga_(0.93)N n-cladding layer 4 (thickness: about 0.6 μm,electron density: 1×10¹⁸/cm³); and a silicon (Si)-doped GaN n-guidelayer 5 (thickness: 80 nm, electron density: 1×10¹⁸/cm³). An activelayer 6 having a multiple quantum well (MQW) structure is formed on then-guide layer 5. The active layer 6 includes six Ga_(0.9)In_(0.1)N welllayers 61 (thickness of each layer: about 2 nm), and fiveGa_(0.97)In_(0.03)N barrier layers 62 (thickness of each layer: about 4nm), the layers 61 and 62 being laminated alternately. The uppermostlayer of the active layer 6 is the Ga_(0.9)In_(0.1)N well layer 61(thickness: about 2 nm). On the active layer 6 are successively formed amagnesium (Mg)-doped GaN p-guide layer 7 (thickness: 80 nm, holedensity: 3×10¹⁷/cm³); a magnesium (Mg)-doped Al_(0.07)Ga_(0.93)Np-cladding layer 8 (thickness: 600 nm, hole density: 3×10¹⁷/cm³); and amagnesium (Mg)-doped GaN p-contact layer 9 (thickness: 200 nm, holedensity: 1×10¹⁸/cm³). A metal nitride electrode 10 of the presentinvention is formed on the p-contact layer 9. An electrode 110comprising vanadium (V) and Al is formed on the n⁺ layer 3.

Next will be described a method for producing the laser diode 100 havingthe aforementioned structure.

The aforementioned laser diode 100 was produced through metal-organicvapor phase epitaxy (hereinafter abbreviated as “MOVPE”). The followinggasses were employed: NH₃, a carrier gas (H₂ or N₂), trimethylgallium(Ga(CH₃)₃, hereinafter abbreviated as “TMG”), trimethylaluminum(Al(CH₃)₃, hereinafter abbreviated as “TMA”), trimethylindium (In(CH₃)₃,hereinafter abbreviated as “TMI”), silane (SiH₄), andcyclopentadienylmagnesium (Mg(C₅H₅)₂, hereinafter abbreviated as“Cp₂Mg”).

Firstly, H₂, NH₃, and TMA were introduced, to thereby form the AlNbuffer layer 2 (thickness: about 30 nm) on the sapphire substrate 1.Subsequently, H₂, NH₃, TMG, and silane (SiH₄) which had been dilutedwith H₂ gas to 0.86 ppm were introduced, to thereby form the silicon(Si)-doped GaN n⁺ layer 3 (thickness: about 5 μm, electron density:1×10¹⁸ /cm³).

After the aforementioned n⁺ layer 3 was formed, H₂, NH₃, TMA, TMG, andsilane (SiH₄) which had been diluted with H₂ gas to 0.86 ppm wereintroduced, to thereby form the silicon (Si)-doped Al_(0.07)Ga_(0.93)Nn-cladding layer 4 (thickness: 600 nm, electron density: 1×10¹⁸/cm³).Subsequently, H₂, NH₃, TMG, and silane (SiH₄) which had been dilutedwith H₂ gas to 0.86 ppm were introduced, to thereby form the silicon(Si)-doped GaN n-guide layer 5 (thickness: 80 nm, electron density:1×10¹⁸/cm³).

Subsequently, N₂ or H₂, NH₃, TMG, and TMI were introduced, to therebyform the Ga_(0.9)In_(0.1)N well layer 61 (thickness: about 2 nm).Subsequently, N₂ or H₂, NH₃, TMG, and TMI were fed, to thereby form theGa_(0.97)In_(0.03)N barrier layer 62 (thickness: about 4 nm). The welllayer 61 and the barrier layer 62 were deposited alternately (number ofdeposition cycles: 5). Thereafter, the uppermost layer; i.e., theGa_(0.9)In_(0.1)N well layer 61 (thickness: about 2 nm), was formed.Thus, the active layer 6 having an MQW structure was formed.

Subsequently, N₂ or H₂, NH₃, TMG, and Cp₂Mg were introduced, to therebyform the magnesium (Mg)-doped GaN p-guide layer 7 (thickness: about 80nm). Subsequently, N₂ or H₂, NH₃, TMA, TMG, and Cp₂Mg were introduced,to thereby form the magnesium (Mg)-doped Al_(0.07)Ga_(0.93)N p-claddinglayer 8 (thickness: about 600 nm). Subsequently, N₂ or H₂, NH₃, TMG, andCp₂Mg were introduced, to thereby form the magnesium (Mg)-doped GaNp-contact layer 9 (thickness: about 100 nm).

Subsequently, the p-contact layer 9, the p-cladding layer 8, and thep-guide layer 7 were uniformly irradiated with electron beams by use ofan electron beam irradiation apparatus. Through irradiation of electronbeams, the hole densities of the p-contact layer 9, p-claddinq layer 8,and p-guide layer 7 became 1×10¹⁸/cm³, 3×10¹⁷/cm³, and 3×10¹⁷/cm³,respectively. Thus, a wafer having a multi-layer structure was produced.

The electrodes are formed in a manner described below.

In order to form an electrode on the n⁺ layer 3, an etching mask wasformed on the p-contact layer 9; a predetermined region of the mask wasremoved; and a portion of the p-contact layer 9, the p-cladding layer 8,the p-guide layer 7, the active layer 6, the n-guide layer 5, then-cladding layer 4, and the n⁺ layer 3, which was not covered with themask, were etched by RIE (reactive ion etching) process employing achlorine-containing gas, to thereby expose the surface of the n⁺ layer3. A vanadium (V) film (thickness: 20 nm) and a film made of at leastone of Al and an Al alloy (thickness: 1.8 μm) were deposited onto theexposed surface of the n⁺ layer 3 by an electron beam evaporationmethod, to thereby form the electrode 110. The metal nitride electrode10 comprising at least one of titanium nitride (TiN) and tantalumnitride (TaN) (thickness: 50 nm) was formed by the aforementionedreactive sputtering on a portion of the p-contact layer 9.

After the electrodes 10 and 110 were formed, a chamber in which thethus-produced laser diode was placed was evacuated to about 1.3×10⁻⁶ Paor less by use of a vacuum pump, and the temperature of the environmentwas risen to about 800° C. and the laser diode was heated for about 15minutes. As a result, alloying of the contact layer 9 and the metalnitride electrode 10 and alloying of the electrode 110 and the n⁺ layer3 were completed.

The value of the contact resistivity of the above-produced electrode 10was measured by means of a transmission line model (TLM) method. As aresult, the value of the contact resistivity was found to be 2×10⁻⁵Ωcm², and the value of the contact resistivity was not increased withpassage of time. The threshold of the laser diode 100 could be reducedby about 5%.

FIG. 4 is a schematic cross-sectional view of a flip-chip-typelight-emitting diode (LED) 101 according to a second embodiment of theinvention, which includes a GaN-based compound semiconductor formed on asapphire substrate 11. A silicon oxide protective layer is notillustrated in FIG. 4.

As shown in FIG. 4, an aluminum nitride (AlN) buffer layer 12(thickness: about 25 nm) is formed on the sapphire substrate 11, and asilicon (Si)-doped GaN high-carrier-concentration n⁺ layer 13(thickness: about 4.0 μm) is formed on the buffer layer 12. An Si-dopedn-type GaN cladding layer 14 (thickness: about 0.5 μm) is formed on thehigh-carrier-concentration n⁺ layer 13.

A light-emitting layer 15 having a multiple quantum well (MQW) structureis formed on the cladding layer 14. The light-emitting layer 15 includessix In_(0.20)Ga_(0.80)N well layers 151 (thickness of each layer: about35 Å) and five GaN barrier layers 152 (thickness of each layer: about 35Å), the layers 151 and 152 being laminated alternately. An Mg-dopedp-type Al_(0.15)Ga_(0.85)N cladding layer 16 (thickness: about 50 nm) isformed on the light-emitting layer 15. A p-type GaN contact layer 17(thickness: about 100 nm) having an Mg density of 1×10¹⁹/cm³ and a holedensity 6×10¹⁷/cm³ is formed on the cladding layer 16.

A metal nitride electrode 18A is formed on the contact layer 17, and anelectrode 18B is formed on the n⁺ layer 13 which is partially exposed byetching. The electrode 18A connected to the contact layer 17 is formedof a metal nitride film (thickness: 50 nm) comprising at least one oftitanium nitride (TiN) and tantalum nitride (TaN). The electrode 18B isformed of a vanadium (V) film (thickness: 200 Å) and a film of at leastone of aluminum (Al) and an aluminum alloy (thickness: 1.8 μm).

The electrodes are formed in a manner described below.

In order to form an electrode on the n⁺ layer 13, an etching mask wasformed on the contact layer 17; a predetermined region of the mask wasremoved; and a portion of the contact layer 17, the cladding layer 16,the light-emitting layer 15, the cladding layer 14, and the n⁺ layer 13,which was not covered with the mask, were etched by RIE (reactive ionetching) employing a chlorine-containing gas, to thereby expose thesurface of the n⁺ layer 13. A vanadium (V) film (thickness: 200 Å) and afilm of at least one of Al and an Al alloy (thickness: 1.8 μm) weredeposited onto the exposed surface of the n⁺ layer 13 through anelectron beam method, to thereby form the electrode 18B.

Subsequently, the metal nitride electrode 18A comprising at least one oftitanium nitride (TiN) and tantalum nitride (TaN) (thickness: 50 nm) wasformed on a portion of the contact layer 17 by means of reactivesputtering.

After the electrodes 18A and 18B were formed, a chamber in which thethus-produced light-emitting diode was placed was evacuated to about1.3×10⁻⁶ Pa by use of a vacuum pump, and the temperature of theenvironment was risen to about 800° C. and the light-emitting diode washeated for about 15 minutes. As a result, the resistances of the p-typecontact layer 17 and the p-type cladding layer 16 were reduced, andalloying of the contact layer 17 and the metal nitride electrode 18A andalloying of the electrode 18B and the n⁺ layer 13 were completed.

The value of the contact resistivity of the above-produced electrode 18Awas measured by means of a transmission line model (TLM) method. As aresult, the value of the contact resistivity was found to be 2×10⁻⁵Ωcm². The operation voltage of the LED 101 was found to be less than 4V, and the operation voltage was not increased with passage of time.

1. A method of forming a p-electrode on a device including a p-typeGroup III nitride compound semiconductor, said method comprising:forming said p-electrode comprising a member selected from a groupconsisting of tantalum nitride (TaN_(x)) and tantalum titanium nitride(Ta_(y)Ti_(1−y)N_(z)) on said p-type Group III nitride compoundsemiconductor by a reactive sputtering with a mixing gas of nitrogen anda rare gas, wherein said p-electrode comprises a mixture of said memberselected from the group consisting of tantalum nitride (TaN_(x)) andtantalum titanium nitride (Ta_(y)Ti_(1−y)N_(z)) and at least one oftitanium nitride (TiN), zirconium nitride (ZrN), niobium nitride (NbN),and tungsten nitride (WN).
 2. A method of forming a p-electrodeaccording to claim 1, wherein a heat treatment is carried out at 700 to1000° C. after said sputtering.
 3. A method of fabricating ap-electrode, having a low ohmic contact resistance, on a group IIInitride compound semiconductor multilayered device, said methodcomprising: forming a p-electrode, which comprises at least one oftantalum nitride and tantalum titanium nitride, on a p-type group IIInitride compound semiconductor layer; and heat treating said p-electrodeat a temperature from 700 to 1000° C., wherein said p-electrodecomprises: said at least one of tantalum nitride and tantalum titaniumnitride; and at least one of titanium nitride, zirconium nitride,niobium nitride, and tungsten nitride.
 4. The method of claim 3, whereinsaid forming comprises sputtering using a mixing gas of nitrogen and arare gas.
 5. The method of claim 3, wherein said heat treating resultsin a contact resistivity, as measured by a transmission line modelmethod, of approximately 2×10⁻⁵ Ωcm².
 6. A method of fabricating ap-electrode, having a low ohmic contact resistance, on a group IIInitride compound semiconductor multilayered device, said methodcomprising: forming a p-electrode, which comprises at least one oftantalum nitride and tantalum titanium nitride, on a p-type group IIInitride compound semiconductor layer by sputtering using a mixing gas ofnitrogen and a rare gas; and heat treating said p-electrode at atemperature from 700 to 1000° C., wherein said heat treating results ina contact resistivity, as measured by a transmission line model method,of approximately 2×10⁻⁵ Ωcm², and wherein said p-electrode comprises:said at least one of tantalum nitride and tantalum titanium nitride; andat least one of titanium nitride, zirconium nitride, niobium nitride,and tungsten nitride.
 7. A method of fabricating a p-electrode accordingto claim 6, wherein said heat treating occurs under vacuum for about 15minutes.